There is known an optical waveguide using an electro-optic crystal such as a LiNbO3 (LN) substrate, a LiTaO2 (substrate) or the like. The optical waveguide is formed by, for example, forming a metal film such as titanium on part of a surface of the substrate and thermally diffusing the same into the substrate. Alternatively, the optical waveguide is formed by patterning the metal film on the substrate and then performing proton exchange in benzoic acid. Also, an optical waveguide device is provided with an electrode for use in control in the vicinity of the optical waveguide. As such an optical waveguide device, there may be mentioned a Mach-Zehnder optical modulator (hereinafter, referred to as “MZ”), for example.
The MZ optical modulator is used in optical devices such as an optical transmitter and the like, and modulates the intensity of light input from an external light source to generate an optical signal. The optical waveguide of the MZ optical modulator has a split section that splits incident light from the light source, a pair of arms through which light waves propagate, and a combining section that recombines the light waves propagating through the arms.
The MZ optical modulator performs modulation by generating an electric field by applying a voltage to an electrode provided above the waveguide and controlling the refractive index of light in the waveguide by utilizing the Pockels effect. By this control, the MZ optical modulator is in an on-state in which the light waves strengthen each other and are then output when the light waves combined in the combining section are in phase, and is in off-state in which the light waves cancel each other and no light is output when the two light waves are in opposite phase (in a state having a phase difference of π).
The ratio of the intensity of output light between the on-state and the off-state is called optical quenching ratio and is known as one of the parameters relating to the communications quality. Although the optical quenching ratio is infinite under the ideal conditions, the optical quenching ratio is actually finite due to the ratio of the powers of light propagating through the pair of arms and the influence of light in a higher-order mode.
As to the MZ optical modulator, there is known an art that uses a voltage adjustment to make the powers of the light waves propagating through the arms equal to each other (see Japanese Laid-Open Patent Application No. 2009-80189, for example). There is known another art that provides a stage that follows the combining section with an output waveguide for a higher-order mode to prevent mode-mismatch light from being mixed in the output light (see Japanese Laid-Open Patent Application No. 2010-237376, for example). There is known yet another art that provides the split section with a portion having a lower refractive index than that of another portion to set the ratio of power splitting in the split section equal to 50% (Japanese Laid-Open Patent Application No. 2011-257634, for example).
There is known a further art that provides a part of the arms with a portion that gradually narrows and becomes gradually wide to absorb stress generated in the waveguide at the time of production of the waveguide and cancel birefringence (Japanese Laid-Open Patent Application No. 7-281041, for example). There is a still further art that provides one of the pair of arms with a portion that is narrower than another portion and provides the other arm with a portion that is wider than another portion to compensate for error in the width of the waveguide and error in the angle thereof that are induced during the manufacturing process (Japanese Laid-Open Patent Application No. 2011-118055, for example).